Method and apparatus for setting pilot signal transmit powers

ABSTRACT

The invention relates to a method and apparatus for setting pilot signal transit powers in a CDMA system. Subscriber units ( 211,213,215 ) perform pilot signal measurement measurements and transmits these to a receiver ( 603 ). A processor ( 605 ) determines a ranking factor for each cell ( 217, 219,221 ) at least partly dependent on the pilot signal measurement and a cell overlap condition of the subscriber unit ( 211, 213, 215 ) during measurements. The processor ( 605 ) ranks the cells ( 217,219,221 ) according to the ranking factor, and sets the pilot signal transmit power of at least one cell in response to the ranking of the cells by adjusting a power amplifier ( 609 ). The cell overlap condition is determined as the handover state of the subscriber unit ( 211,213,215 ) or as the number of cells, which can be received by the subscriber unit ( 211,213,215 ) during the measurement The invention is applicable to 3rd generation cellular communication systems.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for setting pilotsignal transmit powers in a Code Division Multiple Access cellularcommunication system.

BACKGROUND OF THE INVENTION

In a cellular communication system, each of the subscriber units(typically mobile stations) communicates with typically a fixed basestation. Communication from the subscriber unit to the base station isknown as uplink and communication from the base station to thesubscriber unit is known as downlink. The total coverage area of thesystem is divided into a number of separate cells, each predominantlycovered by a single base station. The cells are typically geographicallydistinct with an overlapping coverage area with neighbouring cells. FIG.1 illustrates a cellular communication system 100. In the system, abasestation 101 communicates with a number of subscriber units 103 overradio channels 105. In the cellular system, the base station 101 coversusers within a certain geographical area 107, whereas other geographicalareas 109, 111 are covered by other base stations 113, 115. Some overlapareas 117 can be covered by more than one cell.

As a subscriber unit moves from the coverage area of one cell to thecoverage area of another cell, the communication link will change frombeing between the subscriber unit and the base station of the firstcell, to being between the subscriber unit and the base station of thesecond cell. This is known as a handover. Specifically, some cells maylie completely within the coverage of other larger cells.

All base stations are interconnected by a fixed network. This fixednetwork comprises communication lines, switches, interfaces to othercommunication networks and various controllers required for operatingthe network. A call from a subscriber unit is routed through the fixednetwork to the destination specific for this call. If the call isbetween two subscriber units of the same communication system the callwill be routed through the fixed network to the base station of the cellin which the other subscriber unit currently is. A connection is thusestablished between the two serving cells through the fixed network.Alternatively, if the call is between a subscriber unit and a telephoneconnected to the Public Switched Telephone Network (PSTN) the call isrouted from the serving base station to the interface between thecellular mobile communication system and the PSTN. It is then routedfrom the interface to the telephone by the PSTN.

A cellular mobile communication system is allocated a frequency spectrumfor the radio communication between the subscriber units and the basestations. This spectrum must be shared between all subscriber unitssimultaneously using the system.

One method of sharing this spectrum is by a technique known as CodeDivision Multiple Access (CDMA). In a Direct Sequence CDMA (DS-CDMA)communication system, the signals are prior to being transmittedmultiplied by a high rate code whereby the signal is spread over alarger frequency spectrum. A narrowband signal is thus spread andtransmitted as a wideband signal. At the receiver, the originalnarrowband signal is regenerated by multiplication of the receivedsignal with the same code. A signal spread by use of a different codewill not be de-spread by the receiver but will remain a wide band signaland removed by filtering after the de-spreading operation. In thereceiver, the majority of interference caused by interfering signalsreceived in the same frequency spectrum as the wanted signal can thus beremoved by filtering. Consequently, a plurality of subscriber units canbe accommodated in the same wideband spectrum by allocating differentcodes for different subscriber units. Codes are chosen to minimise theinterference caused between subscriber units typically by choosingorthogonal codes when possible. A further description of CDMAcommunication systems can be found in ‘Spread Spectrum CDMA Systems forWireless Communications’, Glisic & Vucetic, Artech house Publishers,1997, ISBN 0-89006-858-5. Examples of CDMA cellular communicationsystems are IS 95 standardised in North America and the Universal MobileTelecommunication System (UMTS) currently under standardisation inEurope.

Each base station transmits a pilot signal, which can be received by thesubscriber units. A subscriber unit measures the pilot signal level ofthe transmitted pilot signal from a number of surrounding base stations.Each base station furthermore transmits information of neighbouring basestations enabling the subscriber unit to search for the pilot signals ofthese cells. The subscriber unit reports the measured signal values backto the network and the preferred serving cell or cells is chosen basedat least partly on these levels. In the simplest form, the serving cellis chosen as the cell whose pilot signal is received at the highestlevel, as this will maximise the quality of the transmissions betweenthe subscriber unit and the base station with the least cost in terms ofpower. In this case, a subscriber unit served by another cell will behanded over to this cell. In a real communication system, such as UMTS,more complex algorithms are used which may take into account factorssuch as the bit error rate of an ongoing communication, the time(handover margin) etc. However, the measured pilot signal strength isone of the most important parameters considered in determining a servingcell (or cells) whether for hand over or for system access.

Spread spectrum systems offer high capacity by allowing a frequencyreuse factor of one. This means that each cell transmits on the samefrequency. While, this allows maximum usage of the radio spectrum, itcauses each cell to be an interference source to every other cell withinits transmission range. In an interference limited spread spectrumsystem the major loss of available capacity is multiple pilotinterference. This is the case where not only the serving cell and itsadjacent neighbour provide coverage to a mobile between these cells, butother unwanted cells also degrade the area with their signals. Normally,this is taken care of by drive testing and making small incrementalchanges in order to try to reduce these multiple pilot regions. However,this is resource intensive and requires skilled personnel. A secondmethod of reducing these multiple pilot regions is based on simulationsduring the planning stages. However, this solution is impaired by thelack of accurate path-loss propagation tools as well as simulationassumptions of operation performance, traffic distribution e.t.c., whichdo not track mobile performance in the field very well.

There is thus a need for an improved system for setting of pilot signalstransmitted by base stations in a cellular communication network.

SUMMARY OF THE INVENTION

The invention seeks to provide an improved system for setting pilotsignal transmit powers.

Accordingly, there is provided a method of setting pilot signal transmitpowers in a CDMA cellular communication system comprising the steps of:receiving pilot signal measurements from at least one measurementreceiver and from a plurality of locations, the pilot measurementsrelating to pilot signals from more than one cell; for a plurality ofcells, determining a ranking factor for each cell at least partlydependent on the pilot signal measurements and a cell overlap conditionof the measurement receiver during measurements; ranking the cellsaccording to the ranking factor; and setting the pilot signal transmitpower of at least one cell in response to the ranking of the cells.

According to a second aspect of the invention, there is provided anapparatus for setting pilot signal transmit powers in a CDMA cellularcommunication system comprising: means for receiving pilot signalmeasurements from at least one measurement receiver and from a pluralityof locations, the pilot measurements relating to pilot signals from morethan one cell; means for determining for a plurality of cells a rankingfactor for each cell at least partly dependent on the pilot signalmeasurements and a cell overlap condition of the measurement receiverduring measurements; means for ranking the cells according to theranking factor; and means for setting the pilot signal transmit power ofat least one cell in response to the ranking of the cells.

Preferably determining a ranking factor comprises evaluating a pluralityof pilot signal measurements and for each pilot signal measurementdetermining a preference value dependent on the pilot signal measurementand a cell overlap condition of the measurement receiver duringmeasurement; and determining the ranking factor in response to thepreference values of the measurements.

According to a feature of the invention the preference value of eachmeasurement depends on pilot signal measurement of the pilot signal ofthe cell being ranked relative to pilot signals measurements for othercells and on the time between measurement reports for that measurementreceiver.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention is described below, by way ofexample only, with reference to the Drawings, in which:

FIG. 1 is an illustration of a cellular communication system accordingto prior art;

FIG. 2 shows an example of a UMTS cellular communication system;

FIG. 3 illustrates a method of setting pilot signal transmit powers inaccordance with an embodiment of the invention;

FIG. 4 illustrates a specific example of the calculation of a rankingfactor for a cell in accordance with an embodiment of the invention;

FIG. 5 illustrates an example of a ranking of cells according to aranking factor determined in accordance with an embodiment of theinvention;

FIG. 6 illustrates an apparatus for setting pilot signal transmit powerin accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The following description focuses on an embodiment compliant with thecurrent approach for the standardisation of UMTS. However, it will beclear to the skilled person that the invention is not limited thereto.

FIG. 2 shows an example of a UMTS cellular communication system. Anumber of base stations (or in UMTS terminology ‘Node B's’) 201,203,205are connected to a controller (known as a Radio Network Controller, RNC,in UMTS). The RNC is connected to a core network 209, which is connectedto other RNC's and other networks, including the fixed line telephonenetwork. Each of the base stations supports subscriber units 211, 213,215 in their respective cells 217, 219, 221. A subscriber unit 211making a call will establish a communication link 223 with a servingbase station 205. The data from the subscriber unit 211 will becommunicated over this communication link 223 to the base station 205and therefrom routed to the destination, typically through the RNC 207and the core network. Further description of the UMTS CDMA cellularcommunication system can be found in “WCDMA for UMTS, edited by HarriHolma and Antti Toskala, Wiley, 2000, ISBN 0-471-72051-8”

For some areas, the geographical and the network layout is such that asubscriber unit can be served by more than one base station. In CDMAsystems, subscriber units in such an overlap area are typically servedby a plurality of based stations simultaneously. In the example of FIG.2, the subscriber unit 215 is in an overlap area where it can be servedby base station 203 and 205. In this case, communication links are setup between the subscriber unit 215 and the base station 203 as well asbetween the subscriber unit 215 and the base station 205. The data fromthe subscriber unit 215 is transmitted simultaneously over both thesecommunication links and is routed from the base stations 203, 205 to theRNC 207 where they are combined. Likewise, data transmitted to thesubscriber unit 215 is routed to both base station 203 and base station205 and be transmitted from both base stations to the subscriber unit215. In this case, the subscriber unit 215 will combine the receiveddownlink data streams from the two individual communication links.

A subscriber unit, which is very close to a base station 205 willinitially be served by only this base station. As it moves towardsanother base station 203, it will enter the overlap area and acommunication link with this base station 203 will be set up. Thesubscriber unit is thus supported simultaneously by two base stations.As it continues to move towards the base station 203, the subscriberunit will exit the overlap area and the initial communication link withthe first serving base station 205 will be closed down so that thesubscriber unit is only served by the second base station 203. Thissequence is known as a soft handover and a subscriber unit beingsupported by more than one base station is known as being in a softhandover. A soft handover may involve more than two base stations asshown for subscriber unit 213 which has simultaneous communication linkswith all three base stations 201, 203, 205 as it is situated in anoverlap area between three cells 217, 219, 221. A subscriber unit in anoverlap area may only be in soft handover with a subset of the possiblesets or may not be in a soft handover but be served by only one cell.

In order for the communication links to be set up properly, it isnecessary to determine which subscriber units are within the coveragearea of which base stations. To this effect, the base stations transmitpilot signals. A pilot signal is typically transmittedomni-directionally from the base station using a known pilot spreadingcode. This enables subscriber units detecting pilot signal byde-correlating the received signal with the desired pilot spreadingcode. By measuring the signal level of the de-correlated signal areceived signal level of the desired pilot tone can be determined. Thespreading code varies between cells, and therefore the subscriber unitcan independently receive the pilot signal of both the serving cell (orcells) and of neighbour cells. The pilot spreading code for differentcells is communicated to the subscriber units on control channels of theserving cell.

The pilot signal level measurements are used in determining the besttarget cell when accessing the system, i.e. it is used by the subscriberunit to determine which base station to transmit a system access requestto. In addition, the pilot signal level measurements are used todetermine a list of potential handover candidates and which cells shouldbe used for serving the cell, i.e. to which base stations acommunication link should be set up. The set of base stations supportingthe mobile station at any given point in time is known as the activeset. In addition the set of base stations being measured but not activein a soft handover is known as the candidate set.

The transmitted powers of the pilot signals therefore influence wherehandover between cells occur. The stronger the transmitted pilot signalpower, the further away from the base station a handover to the basestation will be instigated, and the lower the transmitted pilot signalpower the closer to the base station the subscriber unit will have to bebefore a hand over to that base station is instigated. The effectivecell size for a base station will thus be dependent on the transmittedpilot signal power and by adjusting the transmitted power the cell andoverlap areas can be modified.

For UMTS, handovers are network originated which means that the decisionto set up or close down a communication link between a subscriber unitand base station is made in the RNC. As the decision is partly dependenton the pilot signal level measured at the subscriber unit, thesubscriber unit transmits measurement reports to the network. Thesemeasurement reports include the measured pilot signal level of theserving cell or cells, as well as other cells, which may be potentialcandidates for soft handover.

Since each cell transmits on the same frequency in a CDMA system eachcell is an interference source to every other cell within itstransmission range. In an interference limited spread spectrum system,the major loss of available capacity is multiple pilot interference.This is the case where not only the serving cell and its adjacentneighbour provide coverage to a mobile between these cells, but otherunwanted cells also degrade the area with their signals. It is thereforedesirable to carefully adjust the pilot signal transmit powers, andthereby the overlap areas, such that capacity of the system is maximisedwhile maintaining sufficient coverage.

FIG. 3 illustrates a method of setting pilot signal transmit powers inaccordance with an embodiment of the invention.

It will be clear to the person skilled in the art that the method can beimplemented in a base station or RNC but could in principle beimplemented in any suitable processing device including beingdistributed over a number of network elements, be implemented in adedicated network element and/or be in the core network. The followingdescription focuses on an implementation in an RNC but it will be clearthat the invention is not limited thereto. Alternative locations includethe Operation and Management Centre or other core element devices.

In step 301 pilot signal measurements are received from at least onemeasurement unit and from a plurality of locations. These pilot signalmeasurements are in the preferred embodiment transmitted from thesubscriber units in an active network. According to the UMTS standard,subscriber units continually measure the received pilot signal levelfrom a number of cells including serving cells and neighbour cells. Thefrequency and thresholds of the measurement reports are howeverconfigurable. Thus, a more periodic and comprehensive pilot set can beconfigured in order to support this algorithm. These levels are reportedback to the base station through measurement report messages. In thepreferred embodiment a large number of measurements are received andprocessed in the following steps 303, 305 and 307.

In the preferred embodiment, the pilot signal measurements aremeasurements of the received pilot signal level but alternatively otherpilot signal measurements or a combination of these can be used. Suchmeasurements include for example pilot signal error rates or therelative order of received signal strength between pilot signalsreceived.

Also the measurements can be made by any suitable measurement receiverand may not necessarily be made in an active system. For example, theinvention is equally applicable to pilot signal measurements obtainedfrom a single dedicated measurement receiver during drive testing of anon-operational network.

The plurality of pilot signal measurements received in step 301 is instep 303 used in determining a ranking factor for a plurality of cells.The ranking factor is determined in response to the pilot signalmeasurements and a cell overlap condition of the measurement receiverduring measurements.

The cell overlap condition of the measurement receiver depends on theoverlap between the cells in the location where the measurement receiveris performing the measurement. In the preferred embodiment, it isdetermined as the number of pilot signals that the measurement receivercan receive over a certain threshold (which may be the noise floor ofthe receiver thereby including all pilot signals that can be received).The cell overlap condition will in this case include all cells containedin the active set (i.e. in an active handover) and in the candidate set(i.e. cells which are received and but not currently active in a handover). Also a measurement receiver may not actively be communicatingwith the communication system but simply measure the pilot signalsreceived at a given location and determine the cell overlap condition asequal to the number of pilot signals received in this area.

In another embodiment, the cell overlap condition is determined as thehandover state of the measurement receiver during measurement. If themeasurement is a subscriber unit in a call, the cell overlap conditionmay simply be determined as the number of cells involved in supportingthe communication i.e. 1 if no handover is ongoing and 2,3,4 etc if asoft handover is ongoing dependent on how many cells are active in thehandover.

In the preferred embodiment, the ranking factor of each cell isdetermined by evaluating a plurality of pilot signal measurements. Foreach pilot signal measurement, a preference value is determined which isdependent on the pilot signal measurement itself and on a cell overlapcondition of the measurement receiver during measurement.

Based on the ranking factor determined for each of the plurality ofcells, the cells are ranked in step 305.

Finally, in step 307 the pilot signal transmit power of the cells is setor adjusted in response to the ranking of the cells.

The method can be applied to the whole network or to a smaller subset ofcells.

Furthermore, the pilot signal transmit power of all cells can be set bythis method or it can be used to set or adjust only a subset of thecells considered.

The method can be iterated and thereby used to dynamically adjust thepilot signal transmit powers in a communication system. It will also beclear to the skilled person that the individual steps are notnecessarily sequential but that they can be performed in parallel orwhen ever sufficient data is available. Also each of the steps can beiterated as frequently as desired independently of the iterationfrequency of the other steps. For example each or all of the steps ofdetermining a ranking factor, ranking the cells and setting an pilotsignal transmit power can be performed whenever a new measurement isreceived. Alternatively each step could for example be performed atpredetermined intervals, such that the step of determining a rankingfactor may be performed at hourly intervals whereas the ranking of thecells and the setting of pilot signal transmit powers may be at dailyintervals in order to minimise the disruption to the system.

The embodiment described thus provides a method for setting the pilotsignal transmit powers in response to measurements of the pilot signalsand the cell overlap condition of the receiver during the measurements.Since the pilot signal measurements relate to the transmitted pilotsignal power and the cell overlap condition relate to the amount ofoverlap between cells at the position of the measurement receiver atmeasurement (and thus relate to pilot signal interference), theinvention provides an efficient method for setting the pilot signaltransmit power such that a desired overlap between cells is obtained.The invention thus provides a method for reducing pilot signalinterference while maintaining coverage.

FIG. 4 illustrates a specific example of the calculation of a rankingfactor for a cell in accordance with a preferred embodiment of theinvention.

For each cell an array is defined having a column for each cell overlapcondition, i.e. a column for the measurement receiver or subscriber unitduring measurement being served by a single cell, another column for thesubscriber unit being in a soft handover with two cells, a third for thesubscriber unit being in handover with three cells etc. In addition themeasurements may relate to cells not involved in a soft handover such ascandidate cells, which are used to fill in the columns as well. Thearray consists of active and candidate cells in order to captureinformation on all possible signals in mobile's environment.

The array is defined having a row for each possible relative pilotsignal strength of the cell being ranked. Hence, the first row relatesto a measurement where the given cell is reported as the strongest pilotsignal level, the second row is where the cell is the second strongestetc.

In the preferred embodiment, when a measurement is received it isevaluated for each cell included in the measurement report what therelative signal strength of that cell is and what the cell overlapcondition is. An entry into the corresponding cell of the array for thatcell is then made by adding a preference value to the current value ofthe cell.

As a specific example a measurement may be received with the pilotsignals levels of cell A,B,C and D of A=−6, B=−8, C=−10, D=−12. In theexample the measurement receiver is in a three way soft handover withcell A being the strongest, cell B the second strongest and cell C thethird strongest. In addition it can receive cell D which is part of thecandidate set and the weakest of the four. Accordingly, a preferencevalue entry is made to the fourth column, first row in the array of cellA, the fourth column and second row for cell B, fourth column and thirdrow for cell C and finally in the fourth column and fourth row for cellD.

The preference value entered or added to the existing value can dependon a number of factors. First of all, desired conditions such asdominance and a limited number of soft handover legs get positivereinforcement, while the relatively weak pilot conditions such as thoseareas where a pilot is low on the list of a multiple way handoff arenegatively weighted.

FIG. 4 illustrates how the preference value accordingly depends on whichcell the entry is in such that cells towards row 1 and column 1 havevery high preference values, whereas cells for increasing column and rownumbers have lower preference values including negative values. Thenegatively weighted pilots get stronger multipliers at the bottom ofmultiple pilot regions so that they can be turned down, increasingcapacity by removing pilots not needed in the area. Pilots that showstrong dominance (i.e. serve subscriber unit without requiring softhandovers with many cells) are increased in order to increase capacityby recognising their ability to provide clean coverage since the basestation transmits less power, and thus less interference, to fulfill thesame quality level of the communication.

In the preferred embodiment of a UMTS system, measurement reports arereceived when a change occurs in the handover state or transitions inthe various thresholds in active set or candidate set for the subscriberunit. Therefore, the time between measurements relate to how long acertain hand over state has occurred and the preference value istherefore determined in relation to this time between measurements, suchthat the longer the interval the higher the preference value a desiredhand over state but the lower the preference value for a non-desiredhand over state.

The ranking factor for a cell is in the preferred embodiment determinedas the summation of the preference values of all cells in the array,i.e. as

${RankingFactor} = {\sum\limits_{n}{\sum\limits_{i}{k_{n,i} \cdot T_{n,i}}}}$wherein n ranges over all cells in the array, i ranges over the numberof measurements contained in each cell n, k is the preference valuerelating to the cell overlap condition and relative strength, and T isthe time between measurements.

In an equivalent implementation, only data related to time betweenmeasurements are stored in the array and the impact on the preferencevalue of the cell overlap condition and relative strength is done byweighting each cell in the array when determining the ranking factor.The ranking factor is thus determined as the summation according to:

${RankingFactor} = {\sum\limits_{n}{k_{n}{\sum\limits_{i}T_{n,i}}}}$

However, as k only depends on the cell overlap condition and therelative pilot signal strength (and is thus independent of i), the twoapproaches are identical.

It should be noted that k can be negative as well as positive such thatdesired conditions improve increases the ranking factor whereasundesired conditions decrease it.

It will be clear to the person skilled in the art, that many otherspecific methods and functions can be used for determining thepreference value and the ranking factor. It will also be clear that manyother factors can be used in determining the preference value includingthe absolute measured pilot signal strength or a pilot signal qualitymeasure.

A ranking factor is determined for each cell of a plurality of cells.The higher the ranking factor, the more the cell serves subscriber unitsin a desired way, i.e. as single serving cells or as hand over cells inhandovers with few cells. A low ranking factor indicates that the cellis involved in handovers with many cells or interferes with other cells.Therefore, the pilot signal transmit power is increased for cells withhigh ranking factors and reduced for cells with low ranking factors.

In the preferred embodiment, the adjustment or setting of pilot signaltransmit powers is subject to further conditions as described below.

Firstly, a lower threshold on the pilot signal transmit power isdetermined for each cell such that a minimum coverage area of that cellis guaranteed. The minimum threshold can be determined by drive testingwhich can be performed under single pilot conditions. This ensures thatthe pilot signals are always sufficiently strong to ensure that thecells overlap such that there are no gaps or holes in the coverage ofthe communication system.

Secondly, the adjustment is subject to an upper threshold on the pilotsignal transmit powers. This upper threshold is set such that the basestation will not power up beyond the capacity of the transmitter poweramplifier. The upper threshold is set taking into account the intendedpower increase of the pilot signals as well as the associated powerincrease of all other transmissions affected by the increased pilotsignal transmit power. Specifically, in UMTS the transmitted power foreach traffic channel depends to some extent on the transmitted pilotpower signal and therefore an upper threshold is set in response to thenumber of traffic channels during max loading of the cell and thedependency of the transmit power of each traffic channel on the pilotsignal transmit power.

FIG. 5 illustrates an example of a ranking of cells according to aranking factor determined in accordance with an embodiment of theinvention.

The table of FIG. 5 shows the pilot number, ranking factor, whether thepilot power can be considered for reduction, the proposed change inpilot power and the pilot powers before and after the adjustment step.Note that the change in pilot power is limited to 2 dBW in a singleadjustment step for this example.

In the example of FIG. 5, it can be seen that the pilot signal transmitpower of the cells having a positive ranking factor is increased with avalue dependent on their ranking factor. It can also be seen that thepilot signal transmit power of most cells having a negative rankingfactor is reduced by a value dependent on the ranking factor. However,for cells having pilot 88 and 152, power cannot be reduced any furtherwithout coverage being reduced below what is acceptable, andconsequently the pilot signal transmit power is maintained unchanged.

In the preferred embodiment, the described method is iterated and thepilot signal transmit powers of a different subset of cells is set inconsecutive iterations dependent on the interdependency between cells.Specifically, if the pilot signal transmit power of a cell is set, theinterference to other cells is changed and the conditions for neighbourcells may change. Therefore, for cells with strong interdependency onlyone cell is adjusted in each iteration. This permits the impact of theadjustment to be taken into account before a decision is made on whetherto change the pilot signal transmit power of the impacted cell.

FIG. 6 illustrates an apparatus 600 for setting pilot signal transmitpower in accordance with an embodiment of the invention.

An antenna 601 receives measurements from subscriber units. The antenna601 is connected to a receiver 603 which despreads, demodulates anddecodes the received measurement data. This data is fed to a processor605, which executes the method as previously described. Pilot data isfed to a transmitter 607, which spreads and modulates the pilot signalfor transmission. The transmitter 607 is connected to an adjustablepower amplifier 609, which is connected to an antenna transmitting thepilot signal.

The processor can be implemented anywhere in the system or can bedistributed between a plurality of components. However, in the preferredembodiment it is situated in the RNC. The processor will typicallyreceive measurement data from a plurality of receivers and will adjustthe transmit power of a plurality of power amplifiers.

An advantage of the invention is that it provides a very efficientsystem for setting pilot signal transmit powers such that pilot signalinterference is reduced while maintaining coverage.

Another advantage of the invention is that the transmit pilot signaltransmit powers can be set based on simple measurements, which can beperformed by standard subscriber units.

Another advantage of the invention is that it can be used in anoperational network and does not require drive testing or simulation.

Another advantage of the invention is that it allows adjustment of thepilot signal transmit power based on the conditions in an activecommunication system. The ranking may thus be based on the conditionsexperienced by subscriber units in an active network taking intoconsideration all in a live system such as geographical distribution ofall subscriber units in the network and the actual traffic distributionused. This obviates the need for many assumptions, which are required indimensioning of a system based on drive tests or simulation.

1. A method of setting pilot signal transmit powers in a CDMA cellularcommunication system comprising the steps of: receiving pilot signalmeasurements from at least one measurement receiver and from a pluralityof locations, the pilot measurements relating to pilot signals from morethan one cell; for a plurality of cells, evaluating the plurality ofpilot signal measurements and for each pilot signal measurementdetermining a preference value dependent on the pilot signal measurementand a handover state of the measurement receiver during measurement, anddetermining a ranking factor for each cell in response to the preferencevalues of the measurements; ranking the cells according to the rankingfactor; and setting the pilot signal transmit power of at least one cellin response to the ranking of the cells.
 2. A method of setting pilotsignal transmit powers as claimed in claim 1 wherein the preferencevalue of each measurement depends on pilot signal measurement of thepilot signal of the cell being ranked relative to pilot signalsmeasurements for other cells.
 3. A method of setting pilot signaltransmit powers as claimed in claim 1 wherein the preference value isdependent on the time interval between measurement reports for thatmeasurement receiver, such that a longer time interval increases thepreference value for a desired handover state but lowers the preferencevalue for a non-desired handover state.
 4. A method of setting pilotsignal transmit powers as claimed in claim 1 wherein the ranking factorof a cell is determined as an averaged value of all the preferencevalues calculated.
 5. A method of setting pilot signal transmit powersas claimed in claim 1 wherein an adjustment of the pilot power dependson the ranking factor associated with the at least one cell.
 6. A methodof setting pilot signal transmit powers as claimed in claim 1 wherein anadjustment of pilot signal power is subject to a first thresholddependent on a minimum coverage area for that cell.
 7. A method ofsetting pilot signal transmit powers as claimed in claim 6 wherein theadjustment of pilot signal power is subject to a threshold higher thanthe first threshold dependent on a maximum power capacity of the poweramplifier serving that cell and dependent upon an intended powerincrease of the pilot signals as well as the associated power increaseof all other transmissions affected by the increased pilot signaltransmit power.
 8. A method of setting pilot signal transmit powers asclaimed in claim 1 wherein the at least one measurement receiver is aplurality of subscriber units operable to communicate with the cellularnetwork.
 9. A method of setting pilot signal transmit powers as claimedin claim 1 wherein the steps are iterated and the pilot signals aredynamically updated during system operation.
 10. A method of settingpilot signal transmit powers as claimed in claim 9 wherein the pilotsignal transmit powers of a different subset of cells is set inconsecutive iterations dependent on the interdependency between cells.11. An apparatus for setting pilot signal transmit powers in a CDMAcellular communication system comprising: means for receiving pilotsignal measurements from at least one measurement receiver and from aplurality of locations, the pilot measurements relating to pilot signalsfrom more than one cell; means for evaluating the plurality of pilotsignal measurements and for each pilot signal measurement determining apreference value dependent on the pilot signal measurement and ahandover condition of the measurement receiver during measurement foreach cell, and determining a ranking factor for each cell of a pluralityof cells, in response to the preference values of the measurements;means for ranking the cells according to the ranking factor; and meansfor setting the pilot signal transmit power of at least one cell inresponse to the ranking of the cells.
 12. An apparatus for setting pilotsignal transmit powers as claimed in claim 11 wherein the preferencevalue of each measurement depends on pilot signal measurement of thepilot signal of the cell being ranked relative to pilot signalsmeasurements for other cells.
 13. An apparatus for setting pilot signaltransmit powers as claimed in claim 11 wherein the preference value isdependent on the time interval between measurement reports for thatmeasurement receiver, such that a longer time interval increases thepreference value for a desired handover state but lowers the preferencevalue for a non-desired handover state.
 14. An apparatus for settingpilot signal transmit powers as claimed in claim 11 wherein the rankingfactor of a cell is determined as an averaged value of all thepreference values calculated.
 15. An apparatus for setting pilot signaltransmit powers as claimed in claim 11 wherein an adjustment of thepilot power depends on the ranking factor associated with the at leastone cell.
 16. An apparatus for setting pilot signal transmit powers asclaimed in claim 11 wherein an adjustment of pilot signal power issubject to a first threshold dependent on a minimum coverage area forthat cell.
 17. An apparatus for setting pilot signal transmit powers asclaimed in claim 16 wherein the adjustment of pilot signal power issubject to a threshold higher than the first threshold dependent on amaximum power capacity of the power amplifier serving that cell anddependent upon an intended power increase of the pilot signals as wellas the associated power increase of all other transmissions affected bythe increased pilot signal transmit power.
 18. An apparatus for settingpilot signal transmit powers as claimed in claim 11 wherein the at leastone measurement receiver is a plurality of subscriber units operable tocommunicate with the cellular network.